The technique of Chirped Pulse Amplification or CPA is at the present time frequently used to amplify short light pulses. Direct amplification of ultrashort pulses (typically lasting from a few tens of femtoseconds to a few picoseconds) will result in deterioration of the optical components of the amplifier well before the desired energy level is reached.
The principle of a CPA chain is described in FIG. 1. The initial pulse P1 output by a laser oscillator OSC is temporally stretched by a stretcher STR in such a way that the duration of the pulse is increased by a factor of typically from 1000 to 5000. The stretched pulse P2 can then be amplified by an amplifier AMP without damaging it. The resulting amplified pulse P3 is then compressed by a compressor CPR in order to reach its initial pulse duration, making it possible to obtain an ultrashort high-power pulse P4.
The process of temporally stretching the incident pulse must be perfectly reversible. Consequently, it is not possible for example to carry out spectral filtering so as to lengthen the duration of the pulse. The technique commonly used consists in utilizing the dispersive properties of dispersive elements of the prism or grating type so as to make the various wavelengths of the incident pulse, the spectral width of which is large (typically a few tens of nanometers) follow an optical path of different length. On exiting the stretcher, the pulse P2 (see FIG. 1) thus has a temporal dispersion of the wavelengths, the shorter-wavelength “blue” (denoted by b) having traveled a longer optical path than the longer-wavelength “red” (denoted by r). To carry out this function, the stretcher comprises, for example, two dispersive elements of the grating type and an afocal system between the gratings for reversing the sign of the dispersion of the grating, or a catadioptric afocal system, allowing only a single grating to be used. Conventionally, the compressor uses dispersive elements very similar to those used in the stretcher and operating with substantially similar angles of incidence, but without the afocal system, so as to make the short wavelengths follow a shorter optical path than the long wavelengths in such a way as to recompress the pulse.
In the field of short-pulse amplifier chains, many applications require the generation of pulses that can be temporally tuned from a few tens of femtoseconds to a few picoseconds without departing from the Fourier limit. In the chains of the prior art, it is necessary, in order to go from amplifying femtosecond pulses to picosecond pulses, to modify the configuration of both the stretcher and the compressor of the CPA chain and to change some of the optical components, especially the gratings. This requires the chain to be realigned. Such a realignment in general requires the intervention of a technical expert.